Poly(phenylene ether) resins (PPE), and particularly the commercially successful engineering plastics based on poly(2,6-dimethyl-1,4-phenylene ether), have been developed over the last thirty years. Despite its excellent physical property characteristics, however, the latter thermoplastic material can not be processed in a conventional manner (e.g., in injection molding operations) due to its high melting viscosity. This disadvantage has been offset to some extend by the discovery that this resin can be blended with polystyrene to form a thermodynamically stable plastic alloy, a rare phenomenon since such polymeric components are generally mutually incompatible. Such alloys of poly(2,6-dimethyl-1,4-phenylene ether) resin with polystyrene (PS), or high impact polystyrene (HIPS), can be readily processed but require the addition of at least about 20 weight percent of the PS or HIPS component to accomplish this end, an additional benefit from such modification being improved impact resistance when HIPS is used.
There is, however, a downside to this modification, since certain physical properties of HIPS, such as heat distortion temperature, are significantly inferior relative to the virgin PPE resin. Attempts to resolve some of these difficulties, as well as to provide poly(phenylene ether) systems having improved mechanical properties, have focused on modification with various rubber components which are dispersed in the poly(phenylene ether) resin.
For example, U.S. Pat. No. 5,365,042 to Cooper et al. describes blends of poly(phenylene ether resins with silicone-grafted EPDM rubber. The resulting compositions exhibit improved ductility and impact strength. Likewise, in U.S. Pat. No. 4,226,761, Cooper et al. disclose a composition comprising a poly(phenylene ether) resin and an alkenyl aromatic resin, such as polystyrene, the latter being modified with a polysiloxane in the form of small rubber-like particles by polymerizing alkenyl aromatic monomer in the presence of the polysiloxane.
European Patent Application 0 369 199 A2 to General Electric Co. teaches thermoplastic molding compositions comprising a poly(phenylene ether) resin which is modified with a multi-stage polyorganosiloxane/vinyl-based graft polymer. These materials are stated to have improved impact resistance, flame resistance and moldability.
In an article published in Makromol. Chem., Suppl. 15, 137-145 (1989), Huang et al. studied the physical and morphological properties of various polyblends of poly(2,6-dimethyl-1,4-phenylene ether) and polydimethylsiloxane in which the former resin was crosslinked in solution to different extents using an amine compound. The siloxanes investigated had relatively low molecular weights (below about 2000) and were capable of forming interpenetrating polymer networks with the poly(phenylene ether) resin in certain instances.
Additionally, organosiloxane compositions in the form of a free-flowing powder prepared from a high consistency "gum-type" polydiorganosiloxane and a reinforcing filler are described in a number of publications, although there is no suggestion to combine these with a given thermoplastic resin, particularly a poly(phenylene ether).
In accordance with the teaching of Link and Scarbel in U.S. Pat. No. 3,824,208, a powdered material is obtained by first reducing the particle size of the polydiorganosiloxane and then mixing the particles with at least 15 parts by weight of a reinforcing filler at a temperature of from 0 to 100.degree. C. and under particular shear conditions.
Japanese Patent Publication No. 2/102007, published on Apr. 13, 1990, teaches pelletizing a high consistency or "gel" type vinyl-containing polydiorganosiloxane and then blending the resultant pellets with a filler. A processing aid is included to prevent a phenomenon referred to as "creping" or "crepe hardening". The resultant composition is then mixed using a high speed rotating blade at 10 to 100.degree. C. to produce a free-flowing powder.
Elastomers prepared from silicone rubber powders according to the above cited teachings of Link and Scarbel and Japanese Patent Publication No. 2/102007 were found to have a number of shortcomings, such as the presence of undesirable gel particles which are discernable to the unaided eye as clear spots when the powdered rubber is combined with a suitable dye and massed into a thin section. This gel problem was essentially overcome by the discoveries of Bilgren et al., as disclosed in a copending application for patent entitled "Storage Stable Organosiloxane Composition and Method for Preparing Same," Ser. No. 790,043, filed on Nov. 12, 1991, now U.S. Pat. No. 5,513,238 assigned to the assignee of the present invention and hereby incorporated by reference. The silicone rubber powder compositions of Mullan et al. have an average particle size of 1 to 1000 microns and are prepared by blending a high consistency polydiorganosiloxane into a quantity of fluidized reinforcing filler that is heated to a temperature of &gt;100.degree. C. to 200.degree. C. prior to, or immediately following, introduction of the polydiorganosiloxane. The resultant rubber powders additionally exhibit excellent storage stability and can be subsequently massed and cured to yield substantially gel-free elastomers having excellent physical properties.
The addition of various rubber compositions, including silicones, to other thermoplastic resin systems has also been investigated. Liang et al., in U.S. Pat. No. 4,888,390, showed that certain rubbers could be used to improve the crack and/or impact resistance of a poly(phenylene sulfide) resin, but there is no suggestion in this patent that a powdered rubber component should be employed.